Disentangling nanotube bundles is important to the efficient use of nanotubes, particularly carbon nanotubes (CNTs). Nanotubes have numerous commercial applications. The unique electrical properties of single wall carbon nanotubes, particularly in the axial direction (excellent electrical conductivity) have opened up uses and potential uses in computers, for example, where significant increases in computing power with decreases in physical size, are being developed. Similarly, improved flexible displays for televisions and computers are being developed with the incorporation of nanotube materials. In medical applications, they have been proposed for use as medical delivery vehicles. However, it is the unusually high mechanical properties (strength and modulus) that have attracted the most interest for nanotube applications. Someday ultra strong carbon nanotubes may be the foundation of a space elevator while their use as reinforcements in composites promises to revolutionize the properties of composite materials. Already, the incorporation of nanotubes in composite systems has lead to significant improvements in composite toughness, strength, stiffness, and conductivity in many laboratories. Commercially, these composite materials have already found limited use in sporting goods (tennis rackets, bicycles, golf clubs), automotive (fuel lines, body parts), and aerospace applications. The difficulty in achieving adequate disentanglement and dispersion currently limits their use in additional applications.
As in any composite material, in a nanotube reinforced composite at least one constituent serves the purpose of providing the reinforcement (nanotubes) while another constituent (e.g. polymeric matrix) serves the purpose of transferring the load between individual reinforcing entities. In order to achieve this reinforcement, it is necessary to maximize the amount of nanotube surface area in direct contact with the material it is reinforcing and to disperse the nanotubes as uniformly as possible throughout the matrix. The nanotube reinforced composites (“NRC”) may include matrix resins such as epoxies, polyesters, polyimides, polyamides, and the like.
The greater the surface area in contact with the material being reinforced and the more uniform the distribution of nanotubes within the material the stronger the composite. However, nanotubes are typically produced in a manner whereby they are not single strands but rather tangled bundles. In the case of single wall nanotubes (SWNTs), the nanotubes are produced as bundles of ropes, caused in part by very strong van der Waals forces. In addition, the high aspect ratios of the carbon nanotubes make it difficult to separate them into individual ropes or tubes. As produced in bundles, the nanotubes offer lowered surface area per unit of weight available for adherence to the material being reinforced. In addition, highly entangled bundles can lead to the nanotubes acting as stress concentrators instead of reinforcements, thus degrading the mechanical properties of the NRC. In other applications, e.g. where nanotubes are incorporated to produce a conductive polymer, adequate dispersibility is required to obtain the continuous and uniform conductivity required throughout the composite. Thus, there is a need for methods to disentangle nanotube bundles into deagglomerated ropes or individual nanotubes.
As discussed in more detail in the next section applicant has found that large pressure differentials create an environment that will separate the bundled nanotubes. More particularly, applicant has found that placing the entangled nanotubes in a high pressure waterjet that is allowed to expand into a zone of lower pressures introduces enough force to disentangle the nanotube bundles without adversely affecting their structure.
The available abstract of Japanese Patent Application Publication No. 150541 (“541 Application”) entitled “Method for Rupturing Carbon Nanotube and [Resulting] Carbon Nanotube” discloses a method for “rupturing carbon nanotubes” but does not disclose disentanglement of nanotubes. The device disclosed in this abstract and, to the limited extent understood, the specification of the '541 Application facilitates this rupturing or breakage of carbon nanotubes by directing multiple streams of water containing nanotubes at each other. These water/nanotube streams are directed through “complicated flow passages of fine tubes.” According to the abstract of the '541 Application, the collision of these streams with each other as they exit the fine tubes and the boundaries of the chamber into which they flow ruptures or breaks the nanotubes. This object of the '541 Application is unlike Applicant's object, namely to debundle and separate the nanotubes to improve their utility, particularly as a reinforcement in composite materials.
A companion Japanese Patent Application Publication No. 2006-016222 (“222 Application”) discloses a device similar to that of the '541 Application for “rupturing” or breaking nanotubes. The primary differences between these two Japanese Applications relate to the structure of the “complicated flow passages” within the devices carrying the water borne nanotubes before they exit the passages and collide. FIG. 2 of the '222 Application schematically illustrates multiple very high pressure (175,000 psi) water streams 14 and 16 which merge as stream 28 after collision and exit the device at discharge port 30. To get the desired degree of rupturing the exit stream for port 30 is split and recycled 10-20 times. There is no disclosure in the '222 Application of any process or means for disentangling nanotubes.